AskDefine | Define spiders

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Noun

spiders
  1. Plural of spider

Extensive Definition

Spiders are predatory invertebrate animals that have two body segments, eight legs, no chewing mouth parts and no wings. They are classified in the order Araneae, one of several orders within the larger class of arachnids, a group that also contains scorpions, whip scorpions, mites, ticks, and opiliones (harvestmen). The study of spiders is called araneology.
All spiders produce silk, a thin, strong protein strand extruded by the spider from spinnerets most commonly found on the end of the abdomen. Many species use it to trap insects in webs, although there are also many species that hunt freely. Silk can be used to aid in climbing, form smooth walls for burrows, build egg sacs, wrap prey, and temporarily hold sperm, among other applications.
All spiders except those in the families Uloboridae and Holarchaeidae, and in the suborder Mesothelae (together about 350 species) can inject venom to protect themselves or to kill prey. Only about 200 species, however, have bites that can pose health problems to humans. Many larger species' bites may be quite painful, but will not produce lasting health concerns.
Spiders are found all over the world, from the tropics to the Arctic, living underwater in silken domes they supply with air, and on the tops of mountains. In 1973 Skylab 3 took two spiders into space to test their web-spinning capabilities in zero gravity.

Morphology

(2) cephalothorax (3) opisthosoma]] Spiders, unlike insects, have only two body segments (tagmata) instead of three: a fused head and thorax (called a cephalothorax or prosoma) and an abdomen (called the opisthosoma). The exception to this rule are the assassin spiders, whose cephalothorax seems to be almost divided into two independent units. Except for a few species of very primitive spiders (family Liphistiidae), the abdomen is not externally segmented. The abdomen and cephalothorax are connected with a thin waist called the pedicle or the pregenital somite, a trait that allows the spider to move the abdomen in all directions. This waist is actually the last segment (somite) of the cephalothorax and is lost in most other members of the Arachnida (in scorpions it is only detectable in the embryos).

Prosoma

All spiders have eight legs, although a few ant-mimicking species use their front legs to imitate antennae, which spiders lack. Their eyes are single lenses rather than compound eyes, ranging from simple light/dark-receptors to eyes rivaling those of a pigeon (some jumping spiders).
They have pedipalps (or just palps), at the base of which are coxae or maxillae next to their mouth that aid in ingesting food; the ends of the palp are modified in adult males into elaborate and often species-specific structures used for mating. Since they have no antennae, they use specialised and sensitive hairs on their legs to pick up scent, sounds, vibrations and air currents.

Sense organs

Spiders usually have eight eyes in various arrangements, a fact that is used to aid in taxonomically classifying different species. Most species of the Haplogynae have six eyes, although some have eight (Plectreuridae), four (eg., Tetrablemma) or even two (most Caponiidae) eyes. Sometimes one pair of eyes is better developed than the rest, or even, in some cave species, there are no eyes at all. Several families of hunting spiders, such as jumping spiders and wolf spiders, have fair to excellent vision. The main pair of eyes in jumping spiders even see in color.
Net-casting spiders have enormous, compound lenses that give a wide field of view and gather available light very efficiently.
However, most spiders that lurk on flowers, webs, and other fixed locations waiting for prey tend to have very poor eyesight; instead they possess an extreme sensitivity to vibrations, which aids in prey capture. Vibration sensitive spiders can sense vibrations from such various mediums as the water surface, the soil or their silk threads. Also changes in the air pressure can be detected in the search for prey.

Respiration and circulation

Spiders have an open circulatory system; i.e., they do not have true blood, or veins to convey it. Rather, their bodies are filled with haemolymph, which is pumped through arteries by a heart into spaces called sinuses surrounding their internal organs.
Spiders have developed several different respiratory anatomies, based either on book lungs, a tracheal system, or both. Mygalomorph and Mesothelae spiders have two pairs of book lungs filled with haemolymph, where openings on the ventral surface of the abdomen allow air to enter and diffuse oxygen. This is also the case for some basal araneomorph spiders like the family Hypochilidae, but the remaining members of this group have just the anterior pair of book lungs intact while the posterior pair of breathing organs are partly or fully modified into tracheae, through which oxygen is diffused into the haemolymph or directly to the tissue and organs. This system has most likely evolved in small ancestors to help resist desiccation. The trachea were originally connected to the surroundings through a pair of spiracles, but in the majority of spiders this pair of spiracles has fused into a single one in the middle, and migrated posterior close to the spinnerets.
Among smaller araneomorph spiders we can find species who have evolved also the anterior pair of book lungs into trachea, or the remaining book lungs are simply reduced or missing, and in a very few the book lungs have developed deep channels, apparently signs of evolution into tracheae. Some very small spiders in moist and sheltered habitats have no breathing organs at all, and instead breathe directly through their body surface. In the tracheal system, oxygen interchange is much more efficient, enabling cursorial hunting (hunting involving extended pursuit) and other advanced characteristics as having a smaller heart and the ability to live in drier habitats.

Digestion

Spiders can only eat their food in liquid form. For this purpose predigestion is carried out both internally and externally to liquefy the tissues of their prey. Some spiders do this by spitting up digestive juices onto prey while chewing it with their chelicerae. The resulting liquefied "soup" is then then sucked up by the spider. Dense combs of hairs around the mouth filter out solids while the spider ingests the liquids. Undigested or uneaten parts of the prey are later discarded. Some spiders do not chew their food, but inject digestive fluids from their stomachs directly into the body of the prey to liquefy the inner tissues and organs. The spider then sucks out the liquefied tissues, eventually leaving the empty outer exoskeleton of the prey.
Many spiders will store prey temporarily. Web-building spiders that have made a shroud of silk to quiet their envenomed prey's death struggles will often leave them in these shrouds and then consume them later.
Spiders are capable of digesting their own silk, so some spiders may eat their used webs. When a spider drops down on a single strand of silk and then returns, it will generally rapidly consume the strand of silk on its way back up.

Spinnerets

The abdomen has no appendages except from one to four (usually three) modified pairs of movable telescoping organs called spinnerets, which produce silk. The suborder Mesothelae is unique in having only two types of silk glands — thought to be the ancestral condition. All other spiders have the spinnerets further towards the posterior end of the body where they form a small cluster, and the anterior central spinnerets on the tenth segment are lost or reduced (suborder Mygalomorphae), or modified into a specialised and flattened plate called the cribellum (parts of suborder Araneomorphae), which produces a thread made up of hundreds to thousands of very fine dry silk fibers resulting in a woolly structure that traps prey. The cribellate spiders were the first spiders to build specialized prey catching webs. Later some groups evolved (called ecribellate) that use silk threads dotted with sticky droplets to capture prey ranging from small arthropods to sometimes even small bats and birds.

Size

Spiders occur in a large range of sizes. The smallest, dwarf spiders of the subfamily Erigoninae, are less than 1 mm (about .05 inches) in body length. The largest and heaviest spiders occur among tarantulas, which can have body lengths up to 90 mm (about 3.5 inches) and leg spans up to 250 mm (about 10 inches).

Coloration

Only three classes of pigment (ommochromes, bilins and guanine) have been identified in spiders, although other pigments have been detected but not yet characterized. Melanins, carotenoids and pterins, very common in other animals, are apparently absent. In some species the exocuticle of the legs and prosoma is modified by a tanning process, resulting in brown coloration. Bilins are found for example in Micrommata virescens, resulting in its green color. Guanine is responsible for the white markings of the European garden spider Araneus diadematus. It is in many species accumulated in specialized cells called guanocytes. In genera such as Tetragnatha, Leucauge, Argyrodes or Theridiosoma, guanine creates their silvery appearance. While guanine is originally an end-product of protein metabolism, its excretion can be blocked in spiders, leading to an increase in its storage.
This stage is differentiated into two sub-stages: the nymph, or juvenile stage and the imago, or adult stage. A spider does not become sexually mature until it makes the transition from nymph to imago.

Sacrificial males

It is a common belief that male spiders, which usually are significantly smaller than the females, are likely to be killed after or during mating, or sometimes even before mating can occur.
Even in some species of widow spiders, which are named exactly for this belief, the male may live in the female's web for some time without being harmed. However, in over 60% of cases the female of one species, the Australian redback spider, kills and eats the male after it inserts its second palpus into the female genital opening Males that 'sacrifice' themselves gain the benefit of increasing their paternity relative to males who do not get cannibalized since they feed the female that will lay and tend the resulting fertilized eggs.
In many other species, males are sometimes killed by females. In at least some of these cases it's likely that the males are simply mistaken as prey. The risk of this happening is greater if the female is hungry. To counter this, some male spiders offer a "bribe" to the female, in form of a fly or other prey, prior to the mating.

Ecology

Spiders have a great range of variation and lifestyle, although all are predatory.
While spiders are generalist predators, in actuality their different methods of prey capture often determine the type of prey taken. Thus web-building spiders rarely capture caterpillars, and crab spiders that ambush prey in flowers capture more bees, butterflies and some flies than other insects. Groups of families that tend to take certain types of prey because of their prey capture methods are often called guilds. A few spiders are more specialized in their prey capture. Dysdera captures and eats sowbugs, pillbugs and beetles, while pirate spiders eat only other spiders. Bolas spiders in the family Araneidae use sex pheromone analogs to capture only the males of certain moth species. Despite their generally broad prey ranges, spiders are one of the most important links in the regulation of the populations of insects.

Behavior

Spiders show a wide variety of behavior, from the ballet-like mating dances of certain jumping spiders to the seeming athletics of bolas spiders snatching their prey. Most diversity comes with the mode of predation, for example whether the spider waits for it in its orb web, or hunts it down.

Predatory techniques

Although spider predatory technique is diverse, as soon as a spider makes contact with its prey, it will usually bite it.
Spiders bite their prey, and occasionally animals that cause them pain or threaten them, for two reasons: First, they inflict mechanical damage, which, in the case of a spider that is as large as or larger than its prey, can be severe. Second, they can inject venom via their hollow fangs. Many genera, such as the widow spiders, inject neurotoxins that can spread through the prey's entire body and interfere with vital body functions. Other genera inject venom that produces tissue damage at the bite location. In the larger victims that do not die from these attacks, painful lesions over a wide area can remain for an extended time. The spitting spiders have modified their poison glands to produce a mixture of venom and sticky substance that works as glue and immobilises the prey.
Although there are no herbivore spiders, some species in the families Anyphaenidae, Corinnidae, Clubionidae, Thomisidae and Salticidae feed on plant nectar. Several spider species are also known to feed on bananas, marmalade, milk, egg yolk and sausages in captivity..

Web types

Tangleweb spiders

Members of this group (family Theridiidae) are characterized by irregular, messy-looking, tangled, three-dimensional (non-sticky) webs, also popularly known as cobwebs, generally low and anchored to the ground or floor and wall. They are commonly found in or near buildings; some build webs in bushes. The spider generally hangs in the center of its web, upside-down. Prey is generally ground-dwelling insects such as ants or crickets, in addition to small flying insects. These include the infamous black widows, the minute happyface spider, and thousands of other species.

Orb web spiders

Spiders in several families (eg., Araneidae, Tetragnathidae, Nephilidae) spin the familiar spiral snare that most people think of as the typical spider web. On average, an orb-weaving spider takes 30 minutes to an hour to weave a web. They range in size from quite large (6+ cm) to very small (<1 cm), but all are quite harmless to humans, beyond the shock entailed from walking into a face-height web and having a large spider dangling from your nose. Many of the daytime hunters have a 'ferocious' appearance, with spines or large 'fangs', but they are almost invariably inoffensive, preferring to drop on a dragline to the ground when disturbed, rather than bite, which can nevertheless be quite painful.

Other types of webs

Some (the Linyphiidae) make various forms of bowl- or dome-shaped webs with or without a flat sheet or a tangled web above or below. Some make a flat platform extending from a funnel-shaped retreat, with generally a tangle of silk above the web. The common northern hemisphere 'funnel-web', 'house' or 'grass' spiders are only superficially similar to the notorious Sydney funnel-web spider, and are generally considered to be quite harmless. Some of the more primitive group Atypidae may make tubular webs up the base of trees, from inside which they bite insects that land on the webbing. These spiders look quite ferocious, but are not generally considered to be particularly dangerous to humans.

Evolution

Trigonotarbids, spider-like arachnids, were among the oldest known land arthropods. Like spiders, they were terrestrial, respired through book lungs, and walked on eight legs with two additional legs adapted to use around their mouth. However, they were not true spiders, not even ancestral to them, but represented independent offshoots of the Arachnida.
True spiders (thin-waisted arachnids) evolved about 400 million years ago, and were among the first species to live on land. They are distinguished by abdominal segmentation and silk producing spinnerets. The Pedipalpi (including whip scorpions) are believed to constitute the sister group to the Araneae.
Most of the early segmented fossil spiders belonged to the Mesothelae, a group of primitive spiders with the spinnerets placed underneath the middle of the abdomen, rather than at the end as in modern spiders (Opisthothelae). They were probably ground dwelling predators of other primitive arthropods. Silk may have been used simply as a protective covering for the eggs, a lining for a retreat hole, and later perhaps for simple ground sheet web and trapdoor construction.
As plant and insect life diversified so also did the spider's use of silk. Spiders with spinnerets at the end of the abdomen (Mygalomorphae and Araneomorphae) appeared more than 250 million years ago, presumably promoting the development of more elaborate sheet and maze webs for prey capture both on ground and foliage, as well as the development of the safety dragline.
By the Jurassic, the sophisticated aerial webs of the orb weaving spiders had already developed to take advantage of the rapidly diversifying groups of insects. A spider web preserved in amber, thought to be 110 million years old, shows evidence of a perfect orb web. It is believed that adhesive capture threads, as opposed to cribellate threads, evolved about 135 million years ago.
The ability to weave orb webs is thought to have been "lost", and sometimes even re-evolved or evolved separately, in different breeds of spiders since its first appearance.

Taxonomy

Almost 40,000 species of spiders (order Araneae) have been identified and are currently grouped into 111 families by arachnologists, but because of difficulties in collecting these often very minute and evasive animals, and because of many specimens stored in collections waiting to be described and classified, it is believed that up to 200,000 species may exist.
The order is composed of three suborders. In the non-venomous primitive Mesothelae, body segmentation is clearly visible, demonstrating the link of spiders with their segmented arthropod ancestors.
The two other suborders, the Mygalomorphae (trapdoor spiders, funnel-web spiders, tarantulas) and the Araneomorphae ("modern" spiders), are sometimes grouped together as Opisthothelae. The latter account for about 94% of all spider species.

Mesothelae

The Mesothelae include the only recent family Liphistiidae. Two more families (Arthrolycosidae and Arthromygalidae) are recognized from fossil evidence only.
The Liphistiidae are burrowing spiders only found in Southeast Asia, China, and Japan with about 90 species in 5 genera. Spiders of this remnant suborder are very rare, and are among the most "primitive" types of spiders in existence.
Recent Mesothelae are characterized by the narrow sternum on the ventral side of the prosoma. Several plesiomorphic characters may be useful in recognizing these spiders: there are tergite plates on the dorsal side and the almost-median position of the spinnerets on the ventral side of the opisthosoma.

Mygalomorphae

The Mygalomorphae are also called the Orthognatha, referring to the orientation of the fangs roughly in line with the body axis. This suborder includes the heavy bodied, stout legged spiders popularly known as tarantulas as well as the dangerous Australasian funnel-web spiders. They have ample poison glands that lie entirely within their chelicerae. Their chelicerae and fangs are large and powerful. Occasionally members of this suborder will even kill small fish, small mammals, etc. Most members of this suborder occur in the tropics and subtropics, but their range can extend farther toward the poles, e.g. into the southern and western regions of the United States and Canada, the northern parts of Europe and south into Argentina and Chile.

Araneomorphae

The Araneomorphae, (previously called the Labidognatha), are often known as the modern spiders.
Araneomorphae are distinguished by fangs that move at a 90 degree angle to the body axis, like a pair of pincers. Most of the spiders that people encounter in daily life belong to this suborder, which makes up 94% of all spider species.
There are approximately 95 families in this suborder, ranging from the minute Patu digua (0.37 mm) to the big and flashy Argiope, from the common orb-weaver spiders to the abstruse assassin spiders, from the reclusive tree trapdoor spiders to the inquisitive jumping spiders.

Creatures often mistaken for spiders

In addition to the true spiders, there are several arachnids commonly mistaken for spiders, but that are not true spiders.
  • Camel spider, a species of solifugid (also commonly called sun-spiders or wind-scorpions), are the source of many urban legends. Although they have no venom the camel spider has been known to attack humans, focusing on exposed skin, and with fangs capable of tearing human flesh. Several myths surround camel spiders, and their size is usually exaggerated. While they are really the size of an adult human hand, myths tell they are as large as the lower half of an adult human leg. Also, they are harmless to humans, and will only attack if disturbed.
  • The daddy long-legs or harvestman is a member of the order Opiliones. These round-bodied arachnids have only two eyes and their heads are fused to their bodies. However, the name "daddy long-legs" is sometimes used to refer to cellar spiders, which have a similar leg shape; these are true spiders. Both are also often said to produce a deadly venom. While the harvestmen do not produce venom at all, the cellar spider's venom is completely harmless to humans. The term daddy long-legs is also used in British English to refer to the Crane fly, which is an insect and not an arachnid at all.

Spiders and people

Spider bites

Most spiders are unlikely to bite humans because they do not identify humans as prey. However, some spiders, even small ones, may bite humans when pinched. For instance, a common species of jumping spider (Family: Salticidae), around 3/8 inch (1 cm) long, when pinched between the folds of a human's palm may inflict a bite that is about as painful as a bee sting. The number of fatalities varies according to author, with some placing the yearly death toll as low as one person per year, worldwide.
Spiders in the world that have been linked to fatalities in humans, or have been shown to have potentially fatal bites by toxicology studies of their venom, include:
Spiders that are never deadly to humans, but that are nonetheless medically significant include:
Spiders that can inflict painful bites (often similar to a bee sting), but whose bites generally do not cause any systemic or long-lasting effects, include:
None of these spiders will intentionally seek out humans, but they should be removed from one's house to avoid accidental injury. Many authorities warn against spraying poisons indiscriminately to kill all spiders, because doing so may actually remove one of the biological controls against incursions of the more dangerous species by ridding them of their competition.
If dangerous spiders are present in your area, be mindful when moving cardboard boxes and other such objects that may have become the shelter of a venomous spider. There is no need to be fearful; just do not grab a spider.

Spiders as food

Spiders, especially larger sorts, are eaten routinely or as a delicacy in various parts of the world, including Cambodia, where fried spider is considered a delicacy, Thailand, the Solomon Islands, and parts of South America, where living wrapped tarantulas are also sometimes taken on trips by certain indigenous tribes.

Arachnophobia

Arachnophobia is a specific phobia, an abnormal fear of spiders. It is among the most common of phobias in certain regions of the world. The reactions of arachnophobics often seem irrational to others (and sometimes to the sufferers themselves). People with arachnophobia tend to feel uneasy in any area they believe could harbor spiders or that has visible signs of their presence, such as webs. If they see a spider they may not enter the general vicinity until they overcome the panic attack that is often associated with their phobia. They may feel humiliated if such episodes happen in the presence of peers or family members. The fear of spiders can be treated by any of general techniques suggested for specific phobias.
Arachnophobia is also the title of a 1990 film, as well as a spin-off video game, in which (fictitious) deadly spiders overrun a small California town.

Spiders in symbolism and culture

There are many references to the spider in popular culture, folklore and symbolism. The spider symbolizes patience for its hunting with web traps, and mischief and malice for its poison and the slow death this causes. It symbolizes possessiveness and storage for its spinning of its prey into a ball and taking it to its burrow (for burrowing species).
Though not all spiders spin gossamer webs, spiders have been attributed by numerous cultures with the origination of basket-weaving, knotwork, weaving, spinning and net making. Spiders are pervasive throughout folklore and mythology. Spinning and binding is evident in the etymologies of the terms religion, yoga, tantra and wyrd.
The Moche people of ancient Peru worshipped nature. They placed emphasis on animals and often depicted spiders in their art.

Footnotes

References

  • Bilger, Burkhard. "Spider Woman". The New Yorker, 5 March 2007, pp. 66–73.
  • The World of Spiders
  • Crompton, John. The Life of the Spider, Mentor, 1950.
  • Hillyard, Paul. The Book of the Spider, Random House, New York, 1994.
  • Kaston, B. J. How to Know the Spiders, Dubuque, 1953.
  • Main, Barbara York. Spiders, Collins (The Australian Naturalist Library), Sydney, 1976.
  • (1998): Cooperative prey capture in the communal web spider, Philoponella raffray (Araneae, Uloboridae). Journal of Arachnology 26: 392–396. PDF
  • Ubick, Darrell; Pierre Paquin, Paula E. Cushing, and Vincent Roth. Spiders of North America: an Identification Manual, American Arachnological Society, 2005.
  • Wise, David H. "Spiders in Ecological Webs." Cambridge University Press. Great Britain: 1993.

Pictures

spiders in Afrikaans: Spinnekop
spiders in Tosk Albanian: Webspinne
spiders in Old English (ca. 450-1100): Ātorcoppe
spiders in Arabic: عنكبوت
spiders in Aymara: Kusikusi
spiders in Min Nan: Ti-tu
spiders in Bosnian: Pauk
spiders in Bulgarian: Паяци
spiders in Catalan: Aranya
spiders in Czech: Pavouci
spiders in Welsh: Corryn
spiders in Danish: Edderkop
spiders in German: Webspinnen
spiders in Estonian: Ämblikulised
spiders in Modern Greek (1453-): Αράχνη
spiders in Spanish: Araneae
spiders in Esperanto: Araneo
spiders in Persian: عنکبوت
spiders in Faroese: Eiturkoppur
spiders in French: Araignée
spiders in Scottish Gaelic: Damhan-allaidh
spiders in Galician: Araña
spiders in Korean: 거미
spiders in Ido: Araneo
spiders in Inuktitut: ᐋᓯᕙᖅ/aasivaq
spiders in Icelandic: Könguló
spiders in Italian: Araneae
spiders in Hebrew: עכבישאים
spiders in Latin: Araneae
spiders in Latvian: Zirnekļi
spiders in Lithuanian: Vorai
spiders in Hungarian: Pókok
spiders in Malayalam: എട്ടുകാലി
spiders in Maltese: Brimba
spiders in Malay (macrolanguage): Labah-labah
spiders in Dutch: Spinnen (dieren)
spiders in Japanese: クモ
spiders in Norwegian: Edderkopper
spiders in Norwegian Nynorsk: Edderkopp
spiders in Narom: Pêtre
spiders in Occitan (post 1500): Araneae
spiders in Uighur: Ömüchük
spiders in Polish: Pająki
spiders in Portuguese: Aranha
spiders in Romanian: Păianjen
spiders in Quechua: Awaq uru
spiders in Russian: Пауки
spiders in Sicilian: Araneae
spiders in Simple English: Spider
spiders in Slovak: Pavúky
spiders in Slovenian: Pajki
spiders in Serbian: Пауци
spiders in Finnish: Hämähäkit
spiders in Swedish: Spindlar
spiders in Tagalog: Gagamba
spiders in Thai: แมงมุม
spiders in Vietnamese: Nhện
spiders in Turkish: Örümcek
spiders in Ukrainian: Павуки
spiders in Yiddish: ספיידער
spiders in Contenese: 蜘蛛
spiders in Chinese: 蜘蛛
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